V.—The Evolution of Instinctive Behaviour
It may be assumed that the fact of evolution is generally admitted. The question of its method is, however, still open to discussion. It is possible that, as some biologists contend, there is an inherent tendency in organic beings to evolve in certain definite directions independently of their relation to the environment. But it is scarcely probable that instinctive behaviour is mainly due to any such inherent tendency—of the nature of which in any case we know but little. Setting this on one side, therefore, we have two hypotheses: first, that instincts are the result of natural selection; secondly, that they are due to the inheritance of acquired habits. These two views we will now proceed to consider.
We have seen that Professor Wundt distinguishes two classes of instinctive acts: first, those which are acquired or have become wholly or partly mechanized in the course of individual life; secondly, those which are connate or have been mechanized in the course of generic evolution. “The laws of practice,” he says,[42] “suffice for the explanation of the acquired instincts. The occurrence of connate instincts renders a subsidiary hypothesis necessary. We must suppose that the physical changes which the nervous elements undergo can be transmitted from father to son.... The assumption of the inheritance of acquired dispositions or tendencies is inevitable if there is to be any continuity of evolution at all. We may be in doubt as to the extent of this inheritance; we cannot question the fact itself.”
Now, the application of the term “instinct,” both to acquired and to connate behaviour, seems to prejudge the question of their genetic connection. And since we have the well-recognized term habits for actions the performance of which becomes automatic through frequency of repetition, we may substitute this term, or the phrase habitual acts, for the “acquired instincts” of Professor Wundt. Modifying, therefore, his statement in accordance with this usage, the fact which, he says, we cannot question is that acquired habits are inherited as congenital instincts. This opinion has long been held: G. H. Lewes regarded instinctive actions as transmitted habits from which the intelligence, through which they were originally acquired, had lapsed. Darwin believed that such inheritance was a factor in the evolution of instinctive behaviour. Romanes distinguished instincts due to this mode of origin as “secondary;” reserving the term “primary” for those attributable to natural selection, and describing those in which both factors co-operate as “instincts of blended origin.” The late Professor Eimer, of Tübingen, going further than either Darwin or Romanes, reverted almost entirely to what we may term the Lamarckian interpretation. “I describe as automatic actions,” he says,[43] “those which, originally performed consciously and voluntarily, in consequence of frequent practice come to be performed unconsciously and involuntarily.... Such acquired automatic actions can be inherited. Instinct is inherited faculty, especially is inherited habit.” In his discussion of the subject Eimer makes no express allusion to primary instincts; but he attributes to lapsed intelligence some of those which were classed by Romanes as primary, and his tendency is to refer all instincts to the same source. “Every bird,” he says, “must, from the first time it hatches its eggs, draw the conclusion that young will also be produced from the eggs which it lays afterwards, and this experience must have been inherited as instinct.” Why, in the first instance, it must draw the conclusion from observation if it inherit instinctive knowledge, is not made clear. But our present purpose is to indicate, not to criticize, Eimer’s position. He claims that “the original progenitors of the cuckoo, when they began to lay their eggs in other nests, acted by reflection and design.” Of the behaviour of mason wasps and their allies, which provide their young with paralyzed but living prey, he exclaims, “What a wonderful contrivance! What calculation on the part of the animal must have been necessary to discover it!” Of the instincts of neuter bees he remarks, “Selection cannot here have had much influence, since the workers do not reproduce. In order to make these favourable conditions constant, insight and reflection on the part of the animals, and the inheritance of these faculties were necessary.” And he concludes, “Thus, according to the preceding considerations, automatic action may be described as habitual voluntary action; instinct, as inherited habitual voluntary action, or the capacity for such action.”
Turning now to the opposite end of the scale of opinion, we find that Professor Weismann, commenting on the supposed inheritance of acquired habit, says,[44] “I believe that this is an entirely erroneous view, and I hold that all instinct is entirely due to the operation of natural selection, and has its foundation, not upon inherited experiences, but upon variation of the germ.” Ziegler and Groos in Germany, Whitman and Baldwin in America, Poulton and Wallace in England, either deny the existence of secondary instincts, due to the inheritance of acquired habits, or question the sufficiency of the evidence adduced in support of such transmission. In their explanation of the manner in which that inherited co-ordination, which is biologically the central fact in instinctive behaviour, has been evolved they rely mainly or entirely on the principle of natural selection.
What, then, were the facts which appeared to Romanes sufficient to justify a belief in the existence of a class of instincts dependent on inherited habit for their origin? He tells us that he only gives a few examples “amongst almost any number” that he could quote. It is certainly unfortunate that, out of more than one hundred and fifty pages devoted to instinct in his work on “Mental Evolution in Animals,” only three[45] are assigned to secondary instincts; or six, if we include one dealing with inherited peculiarities of hand-writing in man, and two showing the force of heredity in the domain of instinct, “whether of the primary or secondary class.” It is true that many pages are devoted to instincts of blended origin, but the co-operation of the Lamarckian factor is here rather assumed than proved. We must, however, be content to take the few examples that are actually given. They are four in number. First, that ponies in Norway are used without bridles, and are trained to obey the voice; and that, as a consequence, a race-peculiarity has been established, for Andrew Knight says that it is impossible to give them what is called “a mouth.” No details are given, and Romanes does not further discuss the evidence. Secondly, Mr. Lawson Tait had a cat which was taught to beg for food like a terrier. All her kittens adopted the same habit under circumstances which precluded the possibility of imitation. Supposing the facts to be correctly reported, and granting that the owners of the kittens, presumably aware of the maternal propensity, did not take some pains to teach the offspring of such a parent to beg (and this does not present much difficulty), one can hardly found a scientific conclusion on so slight an anecdotal basis. Thirdly, instinctive fear is said to be an inherited acquisition; which, fourthly, is lost by disuse. But, as we have already seen, modern investigation has placed this matter of so-called hereditary fear of natural enemies on a different footing. Pheasants, partridges, moor-hens, and wild duck show no fear of a quiet dog. If approached gently, in the absence of their parents, callow wild birds in their nest exhibit little alarm at the slow and gentle approach of man. Mr. Hudson’s opinion has already been quoted, but will bear repetition; it is, “that fear of particular enemies is in nearly all cases the result of experience.” And there is no evidence to show that, in those cases in which it is truly instinctive and not the result of experience, the instinctive behaviour is necessarily due to inherited habit and not to natural selection.
It cannot be said that the evidence for the supposed mode of origin of secondary instincts is sufficiently varied and cogent to carry conviction. On the other hand, there does seem some evidence which points to a different conclusion. When instinctive behaviour follows on a sensory impression, not only is the co-ordination hereditary, but there is an inherited linkage of stimulus and response. Thus in the solitary wasps the sight of the natural prey is followed by the appropriate modes of attack. The Meloë larva springs upon anything hairy. In chicks the sight of a small object at a certain distance initiates the act of pecking. In moor-hens and ducklings the stimulus of water produces the movements concerned in swimming. And so, too, in many other examples of instinctive behaviour, we infer from the observed facts that stimulus and response have an organic connection founded on hereditary links in the nervous system. Now, if such connection were due to inherited habit, we should expect them to be established wherever the experience to which they are related has been constant through many generations. How comes it, then, that the chick does not instinctively respond by appropriate behaviour to the sight of water? How comes it that young birds do not instinctively avoid bees, and wasps, and nauseous caterpillars? If the effects of ancestral experience be hereditary, one would surely expect that in these cases the connection between stimulus and response—a connection which passes into acquired habit—would have become congenital; that the habitual behaviour would have long ago become instinctive. But this does not appear to be the case. And with regard to disuse causing the loss of instinct, how comes it that young chicks swim with well-ordered leg-movements, though swimming is not an act that is habitually performed by the members of their race?
What, then, has the alternative hypothesis of natural selection to advance in explanation of these facts? On this hypothesis instinctive acts have biological value in such degree that they have become congenital through the preservation of adaptive variations. But if this be so, why does not the chick respond instinctively to the sight of that which is so essential to its existence as water to drink? In reply to this question it may be suggested that, under natural conditions, the hen teaches all her chickens to peck at the water, and thus shields them from the eliminating influence which gives rise to natural selection, in the absence of which the habit of drinking in response to the sight of water, though acquired by each succeeding generation of birds, has not become instinctive and congenital. Or, to put the matter from a slightly different point of view, the maternal instincts of the hen protect her chicks from any elimination in this respect; and in the absence of such elimination the habit has not been inherited as instinct. But though the hen can lead her young to peck at the water, she cannot teach them how to perform the complex movements of the mouth, throat, and head in actual drinking. In this matter, therefore, her own instinctive procedure does not shield them from the incidence of that elimination which leads to survival under natural selection. Those chicks would be eliminated which, on pecking the water, failed to respond to the stimulus by the complex behaviour involved in drinking, leaving those to survive in which the response had been congenitally established. Thus it would seem that, when natural selection is excluded, the habit has not become congenitally linked with a visual stimulus; but when natural selection is in operation, the response has been thus linked with the stimulus of water in the bill. Whence we may infer that the co-operation of natural selection is an essential factor in the evolution of instinctive behaviour.
There are, however, cases of instinctive behaviour which may seem too trivial and unimportant to be subject to the sway of natural selection. There are numberless little idiosyncracies of behaviour which seem to be truly instinctive, which are readily recognizable as distinctive traits, but which can hardly be regarded as of sufficient biological value to determine whether the creatures in which they are developed should survive or be eliminated in the struggle for existence. In many cases, however, these serve rather to distinguish the detailed manner of behaviour than its biological end or purpose. In different species natural selection may determine the survival of those whose instinctive behaviour meets a biological need. The relatively unimportant details, differing slightly in each species, are mere adjuncts; and since natural selection deals with each species or inter-generating group separately, the essential behaviour may in each case carry with it the associated differences of manner. We must remember, too, that, as in the matter of structure so in that of behaviour, it is the animal as a whole that is selected for survival; and so long as the whole is adapted to the circumstances of life, the associated differences of form or manner may share in, without doing much to determine, survival. In any case these little instinctive traits, if they are so trivial as to seem of small value from the biological point of view, appear to be too unimportant to have been intelligently acquired as habits.
Let us now consider one or two cases of instinctive behaviour which would fall under Romanes’s category of instincts of blended origin partly due to natural selection, partly to the inheritance of acquired habit. It is the custom of the house martin to build beneath the eaves. Forsaking the ancestral rocky haunts, it has been led to utilize the houses that man has built. This has all the appearance of being due to an intelligent modification of the ancestral instinct; but how far the modification has become through heredity a congenital variation, we do not know. The intelligence which is said to have enabled the martin of the past to adopt this method of nidification is still operative. The nestlings brought up under the eaves would have opportunities for acquiring experience which might lead them to build under similar circumstances. Nest and eaves would be associated in the conscious situation. Nor would the effects of natural selection be necessarily excluded. One may suppose that in the open country, far from rock-shelters, those martins in which there was a congenital tendency to build in house-shelters would bring up their broods and transmit this tendency; while those in which it was absent would either go elsewhere or fail to bring up broods at all. In the absence of fuller knowledge as to the truly instinctive nature of the behaviour, and as to its mode of genesis, we are in large degree at the mercy of conjecture. But in any case the incidence of elimination is not necessarily excluded, and there are, therefore, no grounds for denying that natural selection has been a co-operating factor in the evolution of the instinctive behaviour, if such it be.
It is well known that the lapwing will apparently simulate the actions of a wounded bird, with the object, as it seems, of drawing intruders away from her nest. And such tactics are not restricted to this bird, nor even to one or two species. They are common, no doubt with diversities of detail, to such different birds as grouse, pigeons, plovers, rails, avocets, ducks, pipits, buntings, and warblers. Granting that the behaviour is truly instinctive, it forms a very pretty subject for transmissionists and their critics to quarrel over. “If we seek, as an example,” the transmissionist may exclaim, “an instinct which bears the marks of its intelligent, and therefore acquired origin, this of feigning wounded provides all that we can possibly demand.” “What mode of instinctive behaviour,” the selectionist may ask, “can be adduced which is more obviously useful to the species? Is not this just the kind of procedure which natural selection, if it be a factor at all, must fix upon and perpetuate through the elimination of failures? Those birds which, through congenital variation of behaviour, acted in this way would certainly enable their offspring to escape destruction by enemies, and these would survive to perpetuate the instinct.”
Let us expand the transmissionist position a little further. An extremist, of the type presented by Eimer, would perhaps urge that the lapwing reasons thus: “If I pretend to be wounded, trail my wing, and flutter along the ground, instead of flying off, I shall draw upon myself the intruder’s attention, and lead him to suppose that I shall be easily caught; and if I thus entice him away, my little ones will be saved, and my end gained.” Thus, it may be said, might the bird argue, and then give practical effect to its reasoning. But are we not here attributing to the lapwing powers of ratiocination beyond the capacity of the most intelligent of birds? Are we not assuming a histrionic power, and a realization of the effects on others of its display, which many a human actor might well covet?
“But may not the bird,” it may be urged in reply, “have found by experience, without any elaborate process of abstract reasoning, that the trick is effectual?” In any case it would be experience perilously acquired. Granting that the bird has the wit to try the trick, a little over-acting, a little too much lameness of wing, and she is herself seized and killed; a little under-acting, and the trick fails—her brood is found and destroyed. Does it not seem probable that such experience would be dearly bought, that failure would mean either death to the parent or death to the offspring? And is it not clear that natural selection is thus introduced in any case? And may not the selectionist pertinently ask: “If natural selection is thus introduced as a factor, why halt midway between two hypotheses? Why not take the further step—one by which all the difficulties attending the intelligent acquisition and the biological transmission are alike avoided—of allowing that natural selection exercises, throughout, its influence on congenital variations, and not on acquired modifications of behaviour?”
There is, however, a way in which, when natural selection is operative, intelligence may serve to foster congenital variations of the required nature and direction. We must remember that acquired habits on the one hand, and congenital variations of instinctive behaviour on the other hand, are both working, in their different spheres, towards the same end, that of adjustment to the conditions of life. If, then, acquired accommodation and congenital adaptation reach this end by different methods, survival may be best secured by their co-operation. And the more thorough-going the co-operation the better the chance of survival. There would be a distinct advantage in the struggle for existence when inherited tendencies of independent origin coincided in direction with acquired modifications of behaviour; a distinct disadvantage when such inherited tendencies were of such a character as to thwart or divert the action of intelligence. Thus any hereditary variations which coincide in direction with modifications of behaviour due to acquired habit would be favoured and fostered; while such variations as occurred on other and divergent lines would tend to be weeded out. Professor Mark Baldwin,[46] who has independently suggested such relation between modification and variation, has applied to the process the term “Organic Selection;” but it may also be described as the natural selection of coincident variations.
It may be urged, therefore, that if natural selection be accepted as a potent factor in organic evolution, and unless good cases can be adduced in which natural selection can play no part and yet habit has become instinctive, we may adopt some such view as the foregoing. While still believing that there is some connection between habit and instinct, we may regard the connection as indirect and permissive rather than direct and transmissive. We may look upon some habits as the acquired modifications which foster those variations which are coincident in direction, and which go to the making of instinct.
The net result of a study of instinctive behaviour is to lead us to the conclusion that its evolution runs parallel with the evolution of animal structure. This is perhaps best seen in the case of those insects in which typical instinctive acts are performed by larvæ of wholly different form and structure, though they are stages in the development of the same species. This is exemplified in the cases of Sitaris, Argyromœba, and Leucopsis which have been briefly described. It is probable that in all cases of instinctive action natural selection has been a co-operating factor. Without going so far as to assert with Professor Weismann the “all-sufficiency of natural selection,” we may echo the words of Professor Groos,[47] and say: “Nevertheless, we know no principle except that of selection, and we must go as far as that will take us. Absolute knowledge of such phenomena is unattainable.” And in this conclusion we have the support of Dr. Peckham, who says,[48] “We have found them [the instinctive acts of solitary wasps] in all stages of their development, and are convinced that they have passed through many degrees from the simple to the complex, by the action of natural selection. Indeed, we find in them beautiful examples of the survival of the fittest.”